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100 www.xilinx.com PicoBlaze 8-bit Embedded Microcontroller
UG129 (v1.1.2) June 24, 2008
Appendix : PicoBlaze Instruction Set and Event Reference
R
INTERRUPT Event, When Enabled
The interrupt event is not an instruction but the response of the PicoBlaze microcontroller
to an external interrupt input. If the INTERRUPT_ENABLE flag is set, then a recognized
logic level on the INTERRUPT input generates an Interrupt Event. The action essentially
generates a subroutine CALL to the most-significant instruction address, location 1023
($3FF). The currently executing instruction is allowed to complete. Once the PicoBlaze
microcontroller recognizes the interrupt input, the INTERRUPT_ENABLE flag is
automatically reset. The next instruction in the normal program flow is preempted by the
Interrupt Event and the current PC is pushed onto the CALL/RETURN stack. The PC is
loaded with all ones and the program calls the instruction at the most-significant address.
The instruction at the most-significant address must jump to the interrupt service routine
(ISR).
Pseudocode
; only respond to INTERRUPT input if INTERRUPT_ENABLE flag is set
if ( (INTERRUPT_ENABLE = 1)and (INTERRUPT input = High) ) then
; clear the INTERRUPT_ENABLE flag
INTERRUPT_ENABLE Å 0
; push the current program counter (PC) to the stack
; TOS = Top of Stack
TOS Å PC
; preserve the current CARRY and ZERO flags
PRESERVED_CARRY Å CARRY
PRESERVED_ZERO Å ZERO
; load program counter (PC) with interrupt vector ($3FF)
PC Å $3FF
endif
Registers/Flags Altered
Registers: PC, CALL/RETURN stack
Flags: CARRY, ZERO, INTERRUPT_ENABLE
Notes
The PicoBlaze microcontroller asserts the INTERRUPT_ACK output on the second CLK
cycle of the two-cycle Interrupt Event, as shown in Figure 4-3, page 43. This signal is
optionally used to clear any hardware interrupt flags.
The programmer must ensure that a RETURNI instruction is only performed in response to
a previous interrupt. Otherwise, the PC stack may not contain a valid return address.
Do not use the RETURNI instruction to return from a subroutine CALL. Instead, use the
RETURN instruction.
Because an interrupt event may happen at any time, the values of the CARRY and ZERO
flags cannot be predetermined. Consequently, the corresponding Interrupt Service Routine
(ISR) must not depend on specific values for the CARRY and ZERO flags.